1. Field of the Invention
The present invention relates to a velocity information detecting apparatus for optically detecting information about velocity of relative motion of an object.
2. Related Background Art
An optical Doppler velocity measuring apparatus has been used heretofore for precisely measuring a velocity for example of a rotary drum in a video recorder. Various optical heads of inch size have been developed according to a recent demand of miniaturization.
FIG. 1 is a drawing of optical arrangement to show a conventional laser Doppler velocity sensor.
A collimated beam R emitted from a wavelength stabilized gas laser 1' (wavelength .lambda.) is split by a half mirror HM into two beams R+1 and R-1. The optical axes of principal rays of the beams are bent by respective mirrors M1, M2, and the beams impinge at incident angles +.theta. and -.theta. at a same point on an object L relatively moving at velocity v.
A beam R' scattered from the object and including Doppler signal light is guided toward a light receiving element 6. The light receiving element 6 outputs a signal containing a component with a frequency being a Doppler frequency f as defined by the following formula, which is included in the scattered light. EQU f=(2.multidot.v.multidot.sin.theta.)/.lambda.
A moving velocity of the object can be known by extracting a necessary frequency signal band by a suitable frequency discriminator and counting the frequency of the periodic signal.
The recent demand becomes stronger to obtain a further miniaturized velocity meter. In order to further miniaturize the optical heads down to the millimeter size, new ideas must be introduced for the optical paths or optical systems.